The following is a tabulation of some prior art that presently appears relevant:
U.S. Pat.Pat. No. Kind CodeIssue DatePatentee5,871,518Feb. 16, 1999Blum5,243,998Sep. 14, 1993Silverman et al.5,082,002Jan. 21, 1992Silverman et al.4,640,267Feb. 3, 1987Lawson3,998,209Dec. 21, 1976Macvaugh
U.S. patents application PublicationsPublication NumberKind CodePublication DateApplicant2008/0282993A1Nov. 20, 2008Hoehn2004/0144076A1Jul. 29, 2004Barker2002/0021220A1Feb. 21, 2002Dreyer